The size and sources of the donation will be unavailable for public review until after next Tuesday’s election.

“We believe that message is resonating with voters,” Gomez said.

The money will be used to amplify the “17 Months” ad, which shows a debate clip that the campaign has decided to make its closing argument.

With Markey enjoying sizable leads in public polls, Gomez asked voters to send him to Washington for a 17-month term. After that time, voters could decide if they wanted someone else. Markey, by contrast, has been in the House for 37 years.